Cross-flow filter cartridge

ABSTRACT

The invention relates to improved cross-flow filter cassettes for the filtration of liquid media, which are used in cross-flow filtration devices of varying pump outputs and can be fitted with non-reinforced membranes. The cassettes comprise at least one retentate separator in which the inlets to the open perforations designed to form one type of channel, for example for retentate discharge, are larger than the inlets to the open perforations designed to form another type of channel, for example for fluid feed. In comparison with known cross-flow filter cassettes, the cross-flow filter cassettes of the invention allow retentate flows required to achieve equal filtration output at equal pressure differential on the retentate side at least 1.8 times lower. This significantly reduces the energy input required for pumping to conduct cross-flow filtration.

Pursuant to 35 USC §§371 and 365(b) the priority of Application Nos.PCT/EP 99/03897 filed Jun. 5, 1999 and DE 198 27 4734 filed Jun. 19,1998 is claimed.

BACKGROUND OF THE INVENTION

Cross-flow filtration is generally performed with cassette filtrationsystems wherein several filter cassettes are arranged sequentially as aunit, being pressed between mounting plates that are sealed withsealants in their peripheral edge regions so as to be fluid-tight. Themounting plates are constructed as leading or trailing end plates withcorresponding ports and distributors in channels for feed intake,retentate discharge and permeate discharge. See, for example, WO96/28240 and EP 0 345 209. If the sealants in the peripheral edgeregions involve hard-setting compounds such as polypropylene, the filtercassettes are rendered fluid-tight by using elastomeric sealants. Withpermanently elastic sealing compounds such as silicone, fluid-tightmounting is achieved without the need for additional sealants. Incross-flow filtration, the fluid to be filtered is fed through theleading end plate and the corresponding channels into the feed channelof the filter cassettes for the fluid to be filtered. It flows along thepermselective membrane surfaces and exits the cassette as retentate.Some of it passes through the permselective membranes of the cassetteand is discharged from the system through corresponding channels and thetrailing end plate as permeate. Fluid flows and transmembrane andinternal pressures are regulated by pumps and valves. For optimaloperation of crossflow filtration systems of this kind, pumps must beadjusted to the configuration of the filter cassettes and in particularto the size of the feed channel opening. Transmembrane pressures andfluxes required for efficient filtration must not be reached orexceeded.

Cross-flow filter cassettes are known and disclosed in, for example,U.S. Pat. No. 4,715,955 and DE 34 41 349. Such cassettes are constructedof a multiplicity of adjacent filter cells, each cell consisting ofalternating flat sheet membranes straddled by flat screen members,retentate spacers to form flow channels for the fluid to be filtered,and filtrate spacers to form filtrate-collection channels. Screenmembers and membranes have axially aligned holes, preferably runningperpendicular to their surfaces, to form channels for fluid feed intake,retentate discharge and filtrate discharge.

To protect the membranes from potential mechanical damage in thetransition region of the sealant potentially arising from the membranebeing pressed on or into the retentate and filtrate spacers too firmly,textile-reinforced membranes are often used, wherein which one or bothmembrane surfaces are covered by a textile reinforcement such as a fiberfleece. However, such textile-reinforced membranes generally have areduced flux relative to non-reinforced membranes, thereby loweringtheir filtering capacity. As an alternative, DE 34 41 249 recommendsthat additional protective ring masks and protective frames be includedbetween the screen members. In addition to protection of thenon-reinforced membranes from mechanical damage achieved thereby, thiscauses the flow channel for the fluid to be filtered to be expanded insize, forming a so-called “wide-channel” module. By varying thethickness of the protective ring masks and protective frames, the sizeof the flow channel can be set within certain limits. While theprovision of such ring masks and frames provides a number of advantagessuch as good strength, lower mechanical loads on the membranes, and theability to filter viscous media, at the same time it has thedisadvantage that with the expanded flow channel, the flow-through ratesrequired for optimal filtration of the fluid to be filtered through themembrane surfaces can be achieved only by exceptionally high fluxes. Asa consequence systems equipped with filter cassettes of this kind have ahigh energy demand. Moreover, such cassettes do not achieve optimalperformance in systems equipped with lower pumping capacity.

The use of filter cassettes that have no expanded flow channel such asin a so-called “narrow-channel” module, does have the advantage thathigh fluxes can be readily achieved with low pumping capacity, but atthe same time has the disadvantages of being restricted to the use ofthinner spacers with consequent higher mechanical loads on the membranesand reduced membrane surface area available for filtration. By usingfiner fabrics as spacers, the number of non-filtering points where thereinforcing fabric fibers are applied to the membrane is increased incomparison to thick, coarse fabrics or filter cassettes with built-inprotective ring masks and protective frames. In addition, filtercassettes of this type exhibit low permeability to particles and poorfiltration of viscous media.

It is therefore a principal object of the present invention to provideimproved filter cassettes that can be operated in cross-flow filtrationsystems having a wide variety of pumping capacities. A related object ofthe invention is the provision of improved cross-flow filter cassettesthat can be operated even when equipped with non-reinforced membranes asin the case of a narrow-channel module.

BRIEF SUMMARY OF THE INVENTION

The cross-flow filter cassettes according to the present invention arecharacterized by the fact that they have at least one retentate spacerin which the inlets to the open holes forming one type of channel suchas a retentate discharge channel are larger than the inlets to the openholes forming a second type of channel such as the feed inlet channel.If the cross-flow filter cassettes are connected to the leading and/ortrailing end plates in such a way that greater access to the open holesof the retentate spacers is formed by the channels for the fluid flow, apressure drop arises in the other longitudinal flow channels which haveequal access to the open holes to the channels for the fluid intake andretentate outflow. Due to this higher pressure level in a givenlongitudinal flow channel across the retentate spacer, that longitudinalflow channel expands and exerts a force on adjacent flat sheetmembranes, which in turn expands other longitudinal flow channels,thereby increasing flux for a given feed pressure. Alternatively withequal fluxes, this leads to a lower amount of fluid feed having to bepumped through the filter cassette per unit of time.

In an alternative embodiment of the invention, the membranes are coveredin their edge regions by protective frames that leave the holes open orin the region of their holes by protective ring masks that leave theholes open. This permits mechanically fragile non-reinforced membranesto be installed in the filter cassettes, but at the same time it leadsto expansion of the longitudinal flow channels, requiring greaterpumping capacity, in order to achieve optimal flow through themembranes. But this negative effect is overcome by the invention wherebythe longitudinal flow channels flow are narrowed.

With the cross-flow filter cassettes according to the invention, fluidsare filtered such as liquids, emulsions, suspensions, foods, and drinkssuch as beer, beer seasonings, wine, juice, water, mineral water, andmilk; drinking, process, and waste water; and solutions inpharmaceuticals, medicines, cosmetics, chemistry, biotechnology, genetechnology, electronics, environmental protection, and laboratories.They can be used to separate materials, to disinfect and sterilizesolutions, and to remove pollutants from fluids, for filtration andconcentration of biological solutions, to separate microorganisms suchas bacteria, yeasts, viruses, and cell components, for desalinization ofprotein solutions and other biological media, and for separatingmaterials from ions, macromolecules, and biological molecules.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a plan view of an exemplary retentate spacer of the invention.

FIG. 2 is a plan view of a representative prior art retentate spacer.

FIG. 3 is a schematic section through an exemplary cross-flow filter ofthe cassette.

FIG. 4 is schematic section through a representative prior artcross-flow filter cassette.

FIG. 5 is a graph of the flow of the retentate stream as a function ofthe pressure difference on the retentate side (input pressure lessoutput pressure) with a steady state permeate stream volumetric flow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In a preferred embodiment of the invention, the retentate spacers arearranged with inlets of various sizes to the open holes as externaland/or internal retentate spacers or in a periodic sequence in thefilter cassettes.

Rather surprisingly, it has been found that the cross-flow filtercassettes of the present invention have high filtration capacity insystems that have both high and low pumping capacity if the cassettesare connected to the leading and/or trailing end plates in such a waythat, in the case of high pumping capacity, there is greater access tothe open holes of the retentate spacer(s) that form the channels for theretentate discharge on the trailing side and, in the case of low pumpingcapacity, for the fluid feed intake on the leading side. Thus thecross-flow filter cassettes according to the present invention haveasymmetric flow properties in two possible flow directions. Thisproperty may be exploited in terms of system technology by a flowreversal, for example during cleaning, in order to achieve optimalpressure relationships. Another possibility for utilizing theseasymmetric flow properties consists of starting a cross-flow filtrationsystem with low pressure and gradually increasing the pressure duringthe course of the filtration.

Turning to the drawings, wherein the same numerals designate likeelements, there is shown in FIG. 1, a retentate spacer 2 made up of afabric 1 and having holes 3, 4, 5. Holes 3 are open and form fluid feedchannels 6, while holes 4 are open and form permeate discharge channel7, and holes 5 are open and form retentate discharge channels 8. Openholes 3, 5 are connected so as to be in fluid communication withlongitudinal flow channel or slit 15 (FIG. 3) formed through retentatespacer 2. Open holes 3 have greater access to feed channels 6 than doopen holes 5 to retentate discharge channels 8. Closed holes 4 areenclosed in an elastomeric sealant 9, so applied as to form a smallerinlet 10 to open holes 5 that form retentate discharge channels 8,realized by application of elastomeric sealant 9 which for examplepartially encloses the periphery of open holes 5. For sealing againstinelastic flat sheets or the adjoining protective frame or protectivemasks (not shown), elastomeric sealant compound 9 may extend slightlybeyond the plane of retentate spacer 2. The fluid to be filtered pressesthrough feed channels 6 via the greater inlets to open holes 3 in flowslit 15 (FIG. 3), which is built into the membranes adjacent retentatespacer 2, flows through the fabric 1 on both sides and leaves flow slit15 through inlet 10 to open holes 3 opposite open holes 5 and is thenled away via retentate discharge channels 8. The general flow directionof the fluid to be filtered is indicated by arrow 11. Part of the fluidpenetrates membranes 13 adjacent retentate spacer 2, is collected inlongitudinal flow channels or slits 16 (FIG. 3) and is then led away viaopen holes 4 that form permeate channels 7 in communication withpermeate spacers 14 (FIG. 3).

By way of contrast there is shown a prior art retentate spacer 2′ madeup of a fabric 1, and likewise having holes 3, 4 5′, wherein open holes3 form feed channels 6, closed holes 4 form permeate channels 7, andopen holes 5′ form retentate discharge channels 8. But in this case openholes 3 and 5′ are connected so as to communicate with flow slits 16(FIG. 3) through prior art retentate spacer 2′. Open holes 3 and 5′ haveequal access to feed channels 6 and retentate discharge channels 8.

FIG. 3 shows the structure of a preferred embodiment of the invention.Cross-flow filter cassette 12 consists of an upper and a loser externalretentate spacer 2 of the invention and several internal prior artretentate spacers 2′, membranes 13, and permeate spacers 14. In externalretentate spacers 2 inlets to open holes 3 that form feed channels 6 aregreater than inlets 10 (FIG. 1) to open holes 5 that form retentatedischarge channels 8. Internal prior art retentate spacers 2′ have equalaccess or inlets to open holes 3 and 5′. If the cross-flow filtercassette 12 is connected to the leading and/or trailing end plates insuch a way that the greater access to the open holes 3 of the externalretentate spacers 2 form the feed channels 6, a greater pressuredifferential arises in flow slit 15 than in the other flow slits 16,which have equal access to feed channels 6 and retentate dischargechannels 8. Due to this higher pressure differential flow slit 15expands, which in turn leads to the other flow slits 16 being narrowed,whereby the fluid flow-through rate increases, or with equalflow-through rates, a smaller amount of the fluid is necessarily pumpedthrough the filter cassette per unit of time. This is indicated by thearrows shown in flow slits 15.

In the prior art cross-flow filter cassettes shown schematically in FIG.4 and utilizing the retentate spacers 2′ shown in FIG. 2, equal pressurerelationships prevail in all flow slits 15 and 16 due to the fact thatinlets to open holes 3 and 5′ are of equal size.

EXAMPLE 1

Filtration to concentrate an aqueous protein feed stream containing 5%albumin was performed with a Sartocon®-2 type (Sartorius AG) cross-flowfilter cassette 12 of substantially the same configuration depicted inFIG. 3, consisting of 32 sheets of a 30,000 Dalton molecular weightcutoff ultrafiltration membrane of polyether sulfone having a totalmembrane surface area of 0.7 m², 16 filtrate spacers 14 and 15, amultiplicity of internal retentate spacers 2′ and two external retentatespacers 2. By virtue of application of sealant 9 so as to restrict theinlet to holes 5 the two external retentate spacers 2 had about 20% lessaccess 10 (FIG. 1) to open holes 5 forming retentate discharge channels8 than the access to open holes 3 forming feed channels 6. To maintainan approximately uniform permeate stream, depending on the pressuredifference between input and output pressures, the retentate flows givenin Table 1 were maintained, while the trans-membrane pressure was keptat 2 bars.

TABLE 1 Pressure difference* 1 bar 2 bars 3 bars Retentate flow 445.7750 991.4 (L/h · m²) Permeate flow 110.6 122.6 131.1 (L/h · m²) *Feedside pressure less retentate side pressure

Comparative Example A

Example 1 was repeated with the exception that all the retentate spacerswere of the same prior art configuration as shown in FIG. 2 so that theyall had equal inlets or access to holes 3 and 5′ forming feed andretentate discharge channels 6 and 8, respectively. The data obtainedare set forth in Table 2.

TABLE 2 Pressure difference* 1 bar 2 bars 3 bars Retentate flow 8001347.1 1821.4 (L/h · m²) Permeate flow 113.1 126.9 132.9 (L/h · m²)*Feed side pressure less retentate side pressure

FIG. 5 shows the results obtained from Example 1 and Comparative ExampleA graphically, wherein retentate flow is represented on the leftordinate in L/h·m² and the corresponding permeate flow is represented onthe right ordinate. On the abscissa, the pressure difference isrepresented in bars. FIG. 5 shows that in order to achieve the samefiltering power with the same pressure difference on the retentate sidein comparison with a conventional cross-flow filter cassette, thecross-flow filter cassette of the invention only requires a retentateflow that is smaller by at least a factor of 1.8. This leads to asignificant reduction in the energy needed for pumping to drivecross-flow filtration.

EXAMPLES 2-4

Filtration to concentrate an aqueous protein feed stream containing 5%albumin was performed with three cross-flow filters having theconfiguration shown in FIG. 3 with 1, 2 and 15 retentate spacers of thetype depicted in FIG. 1 and located at various places in the cassette.The inlets to open holes 5 forming retentate channels 8 was about 20%less than the inlets to open holes 3 forming feed channels 6, asschematically depicted in 9, 10 of FIG. 1. All three cassettes were ofthe type used in Example 1, and consisted of 28 sheets of a30,000-Dalton molecular weight cutoff ultrafiltration membrane ofcross-linked cellulose hydrate (Hydrosart®, Sartorius AG), having atotal membrane surface area of about 0.6 m², 14 filtrate spacers and 15total retentate spacers.

Comparative Example B

The filtration of Examples 2-4 was repeated with the same cross-flowfiltration cassette used therein with the exception that all retentatespacers were of the prior art type depicted in FIG. 2, wherein theinlets to open holes 3 and 5′ was equal. The results are summarized inTable 3.

TABLE 3 Number of retentate Retentate Filtrate spacers with Position inflow flow Example reduced access cassette (L/h · m²) (L/h · m²) B 0 notpresent 1340  100 2 1 middle 830 108 3 2 above and 700 110 below 4 15throughout 690 115

From Table 3 it is apparent that the following conclusions may be made:

inclusion of a single inventive retentate spacer in a cassette yieldsthe desired result (Example 2);

making all of the retentate spacers in a filtration cassette of theinventive design does not adversely affect filtration (Example 4); and

use of the retentate spacers of the invention results in a dramaticimprovement in retentate flow and consequent reduction in pumping energyrequired relative to the use of no retentate spacer of the inventivedesign.

It should be understood that the present invention also contemplates theuse of retentate spacers wherein the access to the open holes formingretentate outflow channels may be greater than the access to the openholes forming feed channels (useful in cleaning the filtration cassetteby reverse flow flushing), as well as retentate spacers wherein theaccess to retentate discharge channels is greater or less than theaccess to the feed channels, depending upon the desired application andwhenever a pressure differential in adjoining longitudinal flow channelsis desirable.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding equivalents of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims which follow.

What is claimed is:
 1. In a cross-flow filter cassette comprising amultiplicity of adjacent filter cells wherein each cell consists of theelements (a) at least one retentate spacer forming a longitudinal flowchannel for a fluid to be filtered, (b) a first membrane, (c) at leastone permeate spacer forming a longitudinal permeate-collection channel,and (d) a second membrane wherein elements (a)-(d) have aligned holesforming three types of flow channels selected from (i) fluid feedchannels, (ii) retentate discharge channels and (iii) permeate dischargechannels, the improvement comprising the inclusion of at least onevariable access retentate spacer wherein access to said aligned holesforming one type of flow channel is greater than the access to saidaligned holes forming another type of flow channels.
 2. The cross-flowfilter cassette of claim 1 wherein said first and said second membranesare covered in their peripheral regions by protective frames that leavesaid aligned holes open.
 3. The cross-flow filter cassette of claim 1wherein said first and said second membranes are covered in theirperipheral regions by protective ring masks that leave said alignedholes open.
 4. The cross-flow filter cassette of any of claims 1-3wherein said first and said second membranes are non-reinforced.
 5. Thecross-flow filter cassette of any of claims 1-3 wherein said at leastone variable access retentate spacer is located outside saidmultiplicity of adjacent filter cells.
 6. The cross-flow filter cassetteof any of claims 1-3 wherein said at least one variable access retentatespacer is located within said multiplicity of adjacent filter cells. 7.The cross-flow filter cassette of any of claims 1-3 wherein said atleast one variable access retentate spacer is arranged in said filtercassettes in a periodic sequence.